"Some" is vague/relative
If it only went through one conductor's insulation, and only severed a proportionately few strands (if it's 40-strands, and it severed only 5 for example, a nail may have just shoved them apart more than severing them) then likely nothing would be noticeable (assuming you've removed the nail for prevention of other issues.) If you are running seriously high power on a long run, it might cause some heating problems, but you'd have to be pushing the amps near the limits of the run length and wire gauge, on a constant basis (not something speakers do typically). If you severed 30 out of 40 strands, I'd see about fixing/replacing that damage though. You only need one strand for sound to reach the speaker at some current level, but the fewer strands survived, the more likely you are to reach limits and have problems.
If you feel it's salvageable, get some liquid electrical tape and a straw of something to try and inject into the damaged area and provide some protection from moisture at least, but it's likely that section will corrode over time, especially if sitting as described on top of wood, which seeps moisture for years. So take that into consideration.
Main theater: 14.5'x14.5': LG OLED65C7P; Denon AVR X6400; 11.1 setup with Atmos: Pioneer FS52 (LR), Polk CS20 (c), BS-22 (SR,SL,BR,BL), SP-T22A-LR (x4), SW8-MK2 subwoofer; Samsung UBD-8500 4k BluRay player; XBox One S, XBox 360, Wii, and lots of other odds and ends.
Home studio: 2x Harbinger APS15 speakers, Behringer 8x2 mixer, for various keyboards, guitars, and mic.